1. Field of the Invention
The present invention relates to high-energy electron beams, and more particularly, to a scanned electron-beam x-ray source suitable for computed tomography (CT) imaging systems, such as those used in medical and security applications, and for photon backscattering devices, such as those used for subsurface and through-wall detection.
2. Description of Related Art
It is well known in the art to use a high-energy electron beam to generate x-rays. When high-energy electrons strike a metal of high atomic number, their kinetic energy is converted to x-rays. This principle is employed in x-ray vacuum tubes, which typically use a thermionic cathode to emit electrons, and then form the electrons into a high-energy beam via an anode at a large positive potential relative to the cathode. The beam is directed toward a high-atomic-number target, typically tungsten, and the x-rays resulting from the impact are transmitted out of the tube through a vacuum window. In some cases, an additional accelerator section is incorporated between the anode and target to further increase the energy of the beam.
Applications such as CT systems require that the point of origin of the x-rays be scanned around the object to be imaged. This can be achieved by physically moving the x-ray tube, as in rotating gantry CT machines, or by scanning (deflecting) the electron beam across an elongated target. The latter variety of x-ray tube, typically referred to as a scanned electron-beam source, employs focus and deflection coils mounted externally to the vacuum envelope to control the transmission and the cross-section of the beam as it is swept across a linear or curved target. A collimator is generally used to shape the resulting x-ray beam, which, as in point-source x-ray tubes, is transmitted out of the vacuum via a window.
A conventional scanned electron-beam x-ray source uses an electron gun to produce a beam of electrons that passes through a focus coil used to compress the beam to a small diameter. The beam then enters a large vacuum chamber that is tapered such that it has a narrow end at which the electron beam enters the chamber and a wide end at which is located a heavy metal target that produces x-rays when impinged upon by the electron beam. At the point at which the electron beam enters the vacuum chamber, it passes near a deflection coil that can be used to bend the trajectory of the electron beam and cause it to be selectively directed at various portions of the heavy metal target at the far end of the chamber. By causing the beam to bend through various deflection angles, the deflection coil can direct the electron beam to scan along the heavy metal target, producing x-rays that then exit the vacuum chamber through a window. Thus, by applying an appropriate voltage to the deflection coil, the electron beam can be made to move back and forth to sweep out a V-shape or fan shape, producing x-rays at the point at which the electron beam strikes the heavy metal target.
However, because the electron beam must propagate through a vacuum between the deflection coil and the x-ray target, it is necessary that the vacuum chamber itself be quite large. Furthermore, it is often necessary to limit the maximum deflection angle of the electron beam because the large magnetic fields required to produce large deflection angles can result in aberrations that are undesirable for many applications. Thus, in order to scan across a target of a given length with a limited deflection angle, the target must be placed at a significant distance from the deflection coil, necessitating a large vacuum chamber. As the size and complexity of vacuum systems increase, material and manufacturing costs rise, and reliability can be negatively impacted. Furthermore, the anode or accelerating voltage must be tightly regulated to avoid significant deviations in the spot positional accuracy on its track along the target, since the deflection angle is inversely proportional to the beam velocity.
In many applications of scanning x-ray technology, such as luggage screening, it is important to make the scanner as small as possible, maximize reliability and keep costs down. It is therefore desirable to provide a compact, reliable and low-cost scanning x-ray source.